In 4G, such as Long Term Evolution (LTE) systems, inter-frequency and inter-RAT (Radio Access Technology) mobility measurements and inter-frequency positioning (Observed Time Difference Of Arrival, OTDOA) are carried out by a wireless communication device such as a user equipment (UE) in measurement gaps that are 6 ms long with a periodicity of 40 or 80 ms depending on configuration. These measurement gaps are time periods during which no data transmission is scheduled by the network node (e.g. eNodeB).
FIG. 1 illustrates the impact of a 6 ms measurement gap for downlink (illustrated at upper row of subframes) and uplink (illustrated at lower row of subframes). According to the 3GPP standard for LTE systems (3GPP TS 36.133 V12.6.0 clause 8.1, a user equipment or machine type communication device (MTC) operating on a 4G frequency division duplexing (FDD) carrier shall not transmit on uplink in a subframe that occurs immediately after the measurement gap. Hence the measurement gap, indicated as black subframes in FIG. 1, is 6 subframes in downlink and 7 subframes in the uplink, as illustrated in FIG. 1. Further, a network node, such as a base station, transmitting to the UE in any of the four subframes preceding the measurement gap, these subframes being indicated by hatched subframes in the FIG. 1, cannot get feedback (acknowledgment/negative acknowledgment, ACK/NACK) on the transmitted transport block. This since the feedback is to be sent by the UE four subframes after reception, which in this case would mean during the measurement gap. According to the standard, a non-adaptive retransmission will follow (3GPP TS 36.321 V12.4.0 clause 5.4). As a consequence, the network nodes generally avoid transmitting to the UE in those subframes, meaning that the de facto measurement gap is instead 10 ms on the downlink (the hatched subframes and the black subframes added).
It is similar for the uplink, i.e., due to the measurement gap, the UE cannot get feedback from the network node on whether uplink transmissions were successfully decoded. Here the LTE standard (3GPP TS 36.321 V12.4.0 clause 5.4) stipulates that the UE shall consider a transmitted transport block to have been successfully decoded by the network node. As a consequence, if this is not the case the network node has to schedule the UE to transmit the transport block again, after the measurement gap.
In order for a UE to be able to transmit on the uplink it needs a scheduling grant (SG), which it may request via a scheduling request (SR). The communication resources pointed at by the scheduling grant then appear four subframes after the subframe in which the scheduling grant was received by the UE. Hence if the UE receives a scheduling grant immediately after the measurement gap, it has to wait another four subframes to resume transmissions on the uplink. Hence the uplink outage due to the measurement gap is at least 10 ms, and potentially more if the network node avoids scheduling the UE for uplink transmissions in the four subframes immediately before the gap.
A similar shortcoming due to measurement signaling disturbing uplink transmission arises in case of cell detection, wherein the UE detects synchronization signals transmitted by the network nodes. For instance, for 4G FDD, the repetition period for the synchronization signals is 5 ms and under the assumption that a target cell may be asynchronous usually a minimum of 5 ms+1 Orthogonal frequency-division multiplexing (OFDM) symbol, approximately 5.1 ms, consecutive radio time is used by the UE for each attempt to detect a primary synchronization signal (PSS). Depending on approach for detecting a secondary synchronization signal (SSS) (coherently or non-coherently with respect to PSS), an additional OFDM symbol may need to be collected in order to acquire the pair of PSS and SSS. In 4G TDD (Time Division Duplexing), due to the synchronization signals being laid out differently and having two OFDM symbols inserted between them, up to approximately 5.4 ms contiguous radio time is needed. If less than this radio time is available, the UE will, in worst case, be blind to cells having particular frame timings, particularly if each measurement opportunity is aligned with a raster of 5 ms.
The existing measurement gaps entail a number of drawbacks. For instance, the measurement gaps that are available result in prolonged time to make the measurements. This in turn may have consequences such as e.g. identification of candidate cells (e.g. GSM cells) taking several seconds and the measurement of neighboring cells (e.g. LTE neighboring cells) becoming inefficient due to the few sub-frames that are available within a measurement gap.
Further, for each measurement gap there is about 10 ms when the UE cannot perform uplink transmissions due to either the measurement gap as such, or since after the measurement gap, the scheduling grant that the UE receives is applicable 4 sub-frames later. Moreover, since the UE cannot receive an ACK/NACK on transmissions immediately before the measurement gap, it will in many cases not get scheduled there. This means that the impact of the uplink gap becomes in fact even longer: 14 ms. Given that the measurement gaps may have a repetition rate of 40 ms it is a significant restriction to the scheduling and also has an impact on latency-sensitive applications.